1. Field of the Invention
The present invention is related to the field of downhole nuclear magnetic resonance (NMR) investigation of wellbore fluids. In particular, the invention relates to methods for increasing NMR signal amplitudes in measurements of fluids in downhole environments.
2. Description of the Related Art
Performing measurements on fluid samples is desirable in many oil industry applications. In the prior art, such measurements are typically made by bringing samples to the surface using sealed containers and sending the samples for laboratory measurements. A number of technical and practical limitations are associated with this approach. Such limitations include the limited sample material extractable from a limited number of downhole locations. Also, samples undergo reversible and irreversible changes as a result of the temperature and/or pressure changes while being brought to the surface and during transportation. For example, gases come out of solution, waxes precipitate, and asphaltenes chemically recombine. Irreversible changes eliminate the possibility of ever determining actual in situ fluid properties. Reversible changes are deleterious because they occur slowly and therefore impact sample handling and measurement efficiency. Furthermore, since fluid analysis laboratories are frequently distant from the well site, there are substantial delays—often several weeks—in obtaining results. If a sample is for some reason corrupted or lost during sampling, transportation, or measurement, there is no possibility of returning to the well to replace it.
In view of the foregoing, various methods exist for performing downhole measurements of petrophysical parameters of a geologic formation. Nuclear magnetic resonance (NMR) logging is among the most important methods which have been developed for a rapid determination of such parameters, including formation porosity, composition of the formation fluid, the quantity of movable fluid, permeability and others. At least in part this is due to the fact that NMR measurements are environmentally safe and are unaffected by variations in the matrix mineralogy. In a typical NMR run, a logging tool is lowered into a drilled borehole to measure properties of the geologic formation near the tool. The tool is pulled up at a known rate and measurements are continuously taken and recorded in a computer memory, so that at the end of the run a complete log is generated showing the properties of the geologic formation along the length of the borehole. Alternatively, NMR logging can be done while the borehole is being drilled.
NMR logging is based on the observation that when an assembly of nuclear magnetic moments, such as those of hydrogen nuclei, are exposed to a static magnetic field, they tend to align along the direction of the magnetic field, resulting in a bulk magnetization. The rate at which equilibrium is established in such bulk magnetization upon provision of a static magnetic field is characterized by the parameter T1, known as the spin-lattice relaxation time. Spin-lattice relaxation is caused by energy transfer between the nuclei and the lattice. Another related and frequently used NMR logging parameter is known as the spin-spin relaxation time (also known as transverse relaxation time) T2. Spin-spin relaxation is caused by flip flop processes of neighboring spins. This results in gradual loss of phase coherence of the magnetic moments and hence in a loss of macroscopic magnetization and hence in a loss of NMR signal. Radiofrequency magnetic field bursts (known as “RF pulses”) are used to turn the macroscopic magnetization and to initiate NMR relaxation (see below). It is possible by a succession of RF pulses to generate so-called spin echoes. In fact it is possible to generate with a so-called CPMG sequence of pulses a sequence of spin echoes that decay with the spin-spin relaxation time T2. The NMR echo amplitude of the begin of a CPMG sequence relates directly to the porosity of the earth formation (matrix independent), while both relaxation times provide indirect information about the composition and quantity of the formation fluid, the pore size distribution, and others.
It is not possible to generate a highly homogeneous magnetic field inside the earth formation. For this reason only the NMR signal strength and relaxation can be derived from NMR in the earth formation. There is a need to obtain information about the composition of formation liquids. Formation liquid can be extracted from the formation and analyzed inside the WL or LWD tool in an NMR spectrometer. In this NMR spectrometer the formation liquid sample is NMR-analyzed inside a magnet with comparatively high magnetic field and high magnetic field homogeneity. These magnetic field properties allow chemical shift NMR analysis, not feasible in the formation in situ. Details of this NMR analysis follow in subsequent paragraphs.
The RF frequency ƒ0 needs to meet the NMR resonance condition: ƒ0=γ B0, where γ is the gyromagnetic ratio, a nuclear property specific to the kind of nucleus, and B0 is the externally applied magnetic flux density. A single RF pulse tilts the macroscopic (nuclear) magnetization. The higher the pulse amplitude and the longer the pulse the more will the initial equilibrium magnetization rotate away from the B0 direction. A so-called 90° or π/2 pulse tilts the magnetization from the direction of B0 to a direction perpendicular to B0. After such a pulse the nuclear magnetization precesses with the nuclear resonance frequency ƒ0=γ B0 in a plane perpendicular to the B0 vector. The precessing macroscopic magnetization induces a voltage in the NMR sensor coil, the free induction decay (FID). This NMR signal can be analyzed for frequency distribution. This is done, e.g. by executing a Fourier transformation (FT) of the FID, which will yield a frequency spectrum. In general, any known method to convert time-domain data into a frequency spectrum can be used as an alternative to a FT. If the B0 field homogeneity is sufficient, we will find that the frequency spectrum of an NMR signal of a liquid possesses a fine structure. This is caused by the so-called chemical shift that is caused by electrons. The chemical shift depends on the chemical environment of the nucleus. For this reason “Chemical-shift NMR” also called “High-resolution NMR” has been used for a very long time in laboratory NMR for chemical analysis. Alternatively, Continuous Wave NMR (CW NMR) may be used instead of the pulsed NMR just described. CW NMR sweeps either the magnetic field or the RF frequency over the NMR resonance region observing increased RF absorption at the NMR resonances. This way a frequency spectrum is directly acquired without the need for a Fourier transform. (CW NMR got somewhat out of fashion when the Fast Fourier Transform (FFT) algorithm and powerful digital processors became available.)
Nuclei most often used for Chemical-shift NMR are 1H (protons) and 13C. Chemical shifts of 1H are not more than 10 ppm of the NMR resonance frequency of isolated protons. To make 1H chemical shift NMR work a relative inhomogeneity of the external magnetic field B0 of far less than 1 ppm is required. Carbon-13 NMR (13C NMR) chemical shifts are typically at least an order of magnitude greater and hence require less stringent magnet homogeneity. But 13C has a low natural abundance of only 1% of the total carbon content and a gyromagnetic ratio which is a quarter of that of hydrogen. This results for 13C in a NMR sensitivity that is approximately 6000 times lower than the NMR sensitivity of 1H (at the same B0). Carbon-13 spectroscopy is especially useful in determining the chemical composition of carbon-containing compounds and, as said before, requires not such a very homogeneous magnetic field as 1H spectroscopy.
Some uses of carbon-13 spectroscopy are discussed in prior art. U.S. Pat. No. 5,306,640, issued to Vinegar et al., discusses a method for more accurately determining in-situ oil and brine saturation in porous samples using NMR. Vinegar '640 uses NMR methods for rapid non-destructive analysis of sponge core and obtains information about oil composition and viscosity, which can be obtained simultaneously. The method differentiates between crude oil and water based on frequency-resolved chemical shift NMR spectroscopy of the crude oil and water in a porous medium. The patent of Vinegar '640 uses carbon-13 NMR spectroscopy and a weighted carbon density of the oil to determine a volume of oil.
The method of U.S. Pat. No. 6,111,409, issued to Edwards et al., discusses a method of characterizing a fluid sample withdrawn from an earth formation. Edwards '409 discusses performing nuclear magnetic resonance spin echo measurements on the fluid sample at a nuclear magnetic resonant frequency of carbon-13. Amplitudes of the spin-echo measurements are summed. The summed measurements are spectrally analyzed. The fluid is characterized by determining whether aromatic hydrocarbons are present. This characterization is done by measuring an amplitude of the spectrally analyzed spin echo measurements at about 130 parts per million frequency shift from the carbon-13 resonant frequency. The fluid is also characterized by determining whether aliphatic hydrocarbons are present by measuring an amplitude of the spectrally analyzed spin echo measurements at about 30 parts per million frequency shift.
Carbon-13 NMR signals are typically weak due to the low natural abundance of this nucleus and the low polarizations attainable in thermal equilibrium at normal magnetic fields and temperatures downhole. On the other hand, a high-resolution 13C chemical shift NMR spectrum can be powerful in analyzing the chemical composition of hydrocarbons downhole. There is a need for a method of enhancing NMR signals in a downhole environment. The present invention fulfills that need.